prep o8 237 关于倍数的比较 跪求nn解答

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238. (GWD-4-4)
A new hair-growth drug is being sold for three times the price, per milligram, as the drug's maker charges
for another product with the same active ingredient.
A.  as
B.  than
C.  that
D.  of what      
E.  at which

B.  times+the+n.+than是错误用法；than必须要与比较级搭配
为什么than要和比较级一起 而prep笔记中又说2.  倍数表达：twice (as much as) sth.; X times adj. than; X times as adj. as; double sth.
这两个不是矛盾吗？？

thanks~~